EVALUATION OF THE HOMOGENEITY OF THE WORKING SURFACE OF CAST IRON BRAKE DISCS

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1 p Grzegorz KINAL *, Marta PACZKOWSKA * EVALUATION OF THE HOMOGENEITY OF THE WORKING SURFACE OF CAST IRON BRAKE DISCS OCENA JEDNORODNOŚCI POWIERZCHNI CZĘŚCI ROBOCZEJ ŻELIWNYCH TARCZ HAMULCOWYCH Key words: Abstract Słowa kluczowe: Streszczenie roughness, cast iron, brake discs. This article deals with the one of the most important elements of modern braking systems, which is a brake disc. A brake disc is the one of more stressed parts of the braking system, and its quality and design largely determine the braking performance of the vehicle. The article describes the technology of manufacturing disc brake pads that is important from the point of view of the wear processes occurring between two friction surfaces: the brake disc and the brake pad lining. The research of the cast iron ventilated brake disc surface measured the values of the selected roughness parameters at this site. In the context of measurements, it was also determined to be able to maintain a certain value of selected geometric parameters at a given location for the group of brake discs tested of a specific type and manufactured by a particular manufacturer. The work was carried out in the aspect of the research to create a surface layer to protect the brake discs from the effects of corrosive wear. chropowatość, żeliwo, tarcze hamulcowe. Artykuł dotyczy jednego z ważniejszych elementów współczesnych układów hamulcowych, którym jest tarcza hamulcowa. Tarcze hamulcowe są jednym z bardziej obciążonych elementów układu hamulcowego, a ich jakość i konstrukcja decydują w dużym stopniu o skuteczności hamulców w pojeździe. W części artykułu opisano technologię wytwarzania powierzchni roboczych tarcz hamulcowych ważną z punktu widzenia procesów zużyciowych zachodzących na styku powierzchni ciernej tarczy hamulcowej z powierzchnią cierną okładziny klocka hamulcowego. Wykonane pomiary powierzchni roboczej żeliwnych wentylowanych tarcz hamulcowych pozwoliły ustalić wartości wybranych parametrów chropowatości w tym miejscu. W kontekście pomiarów ustalono także zdolność do utrzymania określonej wartości wybranych parametrów struktury geometrycznej w danym miejscu dla grupy badanych tarcz hamulcowych określonego rodzaju i wyprodukowanych przez określonego producenta. Prace prowadzono w kontekście badań zmierzających do wytworzenia warstwy powierzchniowej zabezpieczającej tarcze hamulcowe przed skutkami zużywania korozyjnego. INTRODUCTION The development of vehicle brake systems has been reported by [L. 1, 2], as a result of two different but simultaneously occurring trends: on the one hand, as a result of the introduction of modern production techniques and, on the other hand, the development of computational and simulation techniques that made the design easier. Undoubtedly, one of the requirements with respect to the braking system is its high reliability. It consists of the reliability of the various components of the system, which forces a stable wear process during operation [L. 3, 4]. One of the components of modern braking systems is the brake disc. Brake discs are the most heavily loaded braking system components, and their quality and design essentially determines the braking performance of the vehicle [L. 5, 6]. At present, grey cast iron with flake or vermicular graphite are used on brake discs [L. 4]. In case of brake discs and drums, the following cast irons are used: with high manganese (up to 1.9%) and phosphorus (up to 0.7%), chromium alloy cast iron (11.5% Cr) with a higher amount of nickel (11.0% Cr, 4.0% Ni), or chromium-nickel-molybdenum cast iron (11.0% Cr, 6.5% Ni, 3.0% Mo. Experimental studies have shown that chromium-nickel cast iron is the most resistant to * Poznan University of Technology, Faculty Machines and Transport, ul. Piotrowo 3, Poznań, Poland, tel.: (0-61) , fax (0-61) , grzegorz.kinal@put.poznan.pl.

2 34 TRIBOLOGIA 6/2017 ISSN abrasive wear [L. 4]. Widespread use of cast iron for the production of brake discs is justified by the simplicity of pre-fabrication technology and relatively low manufacturing costs. The brake disc working part and its state (meaning the surface configuration at its manufacturing stage and during operation) has a great importance in the performance of the disc brake. There is a relationship between the functionality of the surface and the processing conditions, which was exemplified by B. Nowicki [L. 7]. The relationships between physical and functional characteristics and 2D roughness and surface waviness can be determined. It can be seen that, in addition to the altitude and amplitude parameters, horizontal parameters (the mean width of the profile grooves Rsm) and mixed (average slope of the slopes of the profile RΔq) are also important for surface functionality [L. 8]. Newly manufactured brake discs are already corroded as the result of storage. It seems that this problem is also perceptible when selling new cars. In this case, the brake discs mounted on the vehicle waiting to be sold on the lot are also subject to corrosive wear. This particularly applies to the working parts of such shields. Known methods of counteracting corrosion wear in increasing wear resistance of various cast iron components are surface treatment, e.g., nitriding or boronizing [L. 9]. Nevertheless, now, to protect against corrosion on the whole disc (including the work surface), Hawk Performance, for example, produces a thin layer of zinc and aluminium under the patented technology called GEOMET [L. 10]. Brembo also manufactures a water-based coating that protects against the effects of corrosive wear of new brake discs. The research on the improvement of corrosion resistance of cast iron brake discs by nitriding is being carried out at the Department of Road Works and Road Transport of Poznan University of Technology. The research presented in this article is part of preliminary research and aims to evaluate selected parameters of the geometrical structure of the working area of a selected type of cast iron brake disc produced by one manufacturer. These studies are going to be used to evaluate the effect of the nitriding process on the geometric structure of the surface layer. OBJECT AND TEST METHODOLOGY Objects of the presented research are cast iron brake discs manufactured by one of the Polish companies producing brake discs and drums (Fig. 1) [L. 11]. The object of the study was 11 ventilated brake discs used in motor vehicles (Fig. 1). The design of the disc indicates that they have internal ventilation. The disc is a homogeneous cast with the part referred to as the hub of the disc (Fig. 1a). Those discs are designed for use in Opel Astra II (for the Astra Combi 1.3 diesel engine with 66kW) and Opel Zafira. a) b) Fig. 1. View of an exemplary brake disk surface roughened with convex side (a) marked as "A side" of the disc and a concave side (b) marked as "B side" Rys. 1. Widok przykładowej tarczy hamulcowej poddanej pomiarom chropowatości powierzchni pokazujący stronę wypukłą (a) oznaczoną jako strona A tarczy oraz stronę wklęsłą (b) tarczy oznaczoną jako strona B Subsequently, precise markings were applied to each of the test wheels and profilograms were made in the disc area (Fig. 2). Surface profiles were made using a contact profilometer. On the marked discs in the selected places, measurements of 2D surface stereometry in the working part were made in accordance with PN- EN 10049: standard. In each of three selected places on the brake disc surface on both sides: "A" and "B," three profiles were made (Fig. 2). In addition, three more profiles were made perpendicular to the direction of earlier performed profiles (Fig. 2). The lengths of the profiles were fixed at 56 mm. The spacing between successive profiles was also constant and was 5 mm. Fig. 2. Diagram of brake disc with schematically marked location, number, and direction of surface roughness profiles on one side of the disc Rys. 2. Rysunek tarczy hamulcowej ze schematycznie zaznaczonym miejscem, liczbą oraz kierunkiem wykonania profilogramów chropowatości powierzchni na jednej ze stron tarczy This concerns both types of profiles (in the radial direction of the disc as well as the perpendicular direction of the brake discs. The choice of the place of the performance of a given group of profiles was dictated by the distribution of these groups as 120 on the disc working surface relative to the centre of the disc. Surface roughness measurements (2D surface geometry) were

3 35 carried out with a ZAISS contact profilometer equipped with induction transducers and SUFORM with SAJD METROLOGIA software allowing the measurement and analysis of deviations of straightness and surface roughness. The measurement tip travel velocity during measurement was 0.1 mm / s (the device capacity is from 0.01 mm / s to 1 mm / s). The elementary distance was determined (λc = 0.8 mm), while the distance Ln was adequately 4 mm. Roughness filtration was used in the analysis of the roughness results. The obtained results of the measurements were used to determine the basic parameters of the roughness profile, i.e. Ra and Rz parameters. All tests were performed using the devices located in the Laboratory of Stability of the Poznan University of Technology Institute of Machinery and Motor Vehicles. RESULTS AND ANALYSIS RESULTS The area of the tested brake discs (referred as the working surface) shows the surface grinding traces for all of the tested discs. a) b) the mean values of the Ra and Rz parameters for the individual brake discs, the possibility of differences in the test parameters is observed for both sides of tested brake discs (Figs. 4 and 5). When comparing the average values of the parameter Ra determined separately for the "A" and "B" side of the brake disc, differences in the test parameter are observed only for disc No 10 (Fig. 4). When comparing the mean values of the Rz parameter determined separately for the "A" and "B" side of the particular brake discs, differences in the test parameter are observed only for disc No 5 (Fig. 4). The remaining single discs do not show any difference in mean Ra and Rz values in case of one side to the other (Figures 4 and 5). There are also no differences in mean values of the roughness parameters Ra and Rz one side to the other, taking into account the whole group of eleven tested brake discs (Figs. 4 and 5). Based on the results of the research, it was found (in the 220 profile studies) that the mean value of the roughness parameters (Ra and Rz) tested of the working surface of the examined discs, taking into account both sides of all brake discs, was Ra = 2.2 ± 0.25 μm and Rz = 16.4 ± 3.4 μm. On the basis of the Figures 4 and 5, we can also conclude that there are also differences in the roughness parameters tested for each brake disc. The results show that the difference between the mean values of the Ra and Rz parameters in the brake disc working areas can a) Fig. 3. Brake Disc Working surface of No. 7 break discs (B side) in the illustrated view (a) and magnified (b) Rys. 3. Powierzchnia robocza tarczy hamulcowej nr 7 (strona B ) (a) w widoku poglądowym oraz (b) przy zbliżeniu tej samej powierzchni Observations of the working surface of the brake discs show that there are regular scratches. The axes of those scratches are aligned with each other under different angles, thus forming a grid (Figs. 3a and b). The angles of intersection of the scratches on the working surface is diversified (in the range from about 130 in the immediate vicinity of the hub to about 85 on the other end of the brake disk surface (Fig. 3a)). In the middle of the working surface of the disc, the angle between the grooves was about 98. The depth of the resulting grooves cross each other. The scratch structure is similar to the plateau effect. No evidence of corrosive wear was observed on any surfaces of the examined discs after their delivery in the manufacturer's original packaging. The discs were covered with a thin layer of most likely a solid lubricant. Based on surface profiles, selected parameters of roughness Ra and Rz were determined. Data was collected in tables and analysed. Taking into account b) Fig. 4. Changes in the mean value of the Ra parameter for the individual tested brake disc on its working surfaces for both sides. Confidence intervals were set for mean values on significance level 0.1 Rys. 4. Zmiany wartości średnich parametru Ra dla poszczególnych badanych tarcz hamulcowych na powierzchni roboczej dla obu badanych stron tarczy. Przedziały ufności ustalono dla wartości średniej na poziomie istotności 0,1

4 36 TRIBOLOGIA 6/2017 ISSN a) the radial direction of the disc) and Ra = 2.2 ± 0.11 μm (if the profile is perpendicular to the previous profile). CONCLUSIONS b) Fig. 5. Changes in the mean value of the Rz parameter for the individual tested brake disc on its working surfaces for both sides. Confidence intervals were set for mean values on significance level 0.1 Rys. 5. Zmiany wartości średnich parametru Rz dla poszczególnych badanych tarcz hamulcowych na powierzchni roboczej dla obu badanych stron tarczy. Przedziały ufności ustalono dla wartości średniej na poziomie istotności 0,1 be considered taking into account both sides of the test discs. This may indicate that the manufacturer makes brake discs that differ in the surface roughness defined by the Ra and Rz parameters of the working surfaces. The profiles made in two perpendicular directions (Fig. 2, with crossed directions of the profiles), after the parameter Ra, indicate that, in some cases, i.e. Disc 1 side A, the differences in the mean value of the parameter Ra depend on the direction of the measurement (for example, Ra = 2.5 ± 0.36 μm (if the profile is made in Based on the conducted research, the following conclusions and observations can be made: Observations of the working surfaces of the tested brake discs show that they have been subjected to sanding (it seems to be a plateau effect), with crossings of the scratching traces in the range of 85 to 145. In the group of tested brake discs, only one of the 11 discs in its working surfaces showed differences in the mean value of the Ra and Rz parameters when comparing the those parameters on both sides of the disc. There may be differences in the mean value of the parameter Ra measured radially to the axis of the disc and perpendicular to it. There are differences in the mean values of the Ra parameter between the various tested brake discs, in the case of their working surfaces, taking into account both sides of the tested disc. There are differences in the average values of the Rz parameter between the various tested brake discs in case of their working surfaces, taking into account both sides of the test disc. The working surfaces of the tested brake discs are characterized by Ra = 2.2 ±0.25 μm roughness. The working surfaces of the tested brake discs are characterized by Rz = 16.4 ±3.4 μm roughness. It should be emphasized that the analysis of the gathered roughness test results of the individual brake discs has provided information on the condition of the surface in terms of its preparation for their operation, as is currently taking place in the case of vehicles where such discs are mounted. However, the conclusions of the study will primarily serve to compare the changes of this state after the surface treatment processes planned in further research. As a consequence of the work carried out, the results will be a basis for the evaluation of the brake discs behaviour during of the determining their utility values under tribological and laboratory tests. REFERENCES 1. Wojciechowski A., Rudnik D., Pietrzak K., Sobczak J., Nowak A.: Nowa generacja tarcz hamulcowych z materiałów kompozytowych, IV Seminarium Kompozyty 2000 Teoria i praktyka, Częstochowa 2000, s Polska Norma PN-85/H Odlewy. Podział i terminologia wad. 3. Szczerek M., Wiśniewski M.: Tribologia i trybotechnika. Wydawnistwo Instytutu Technologii Eksploatacji. Radom Wojciechowski A., Sobczak J.: Kompozytowe tarcze hamulcowe pojazdów drogowych. Wydawnictwo Instytut Transportu Samochodowego. Warszawa 2001.

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